This invention relates to a Raman Laser and in particular, but not exclusively to a Raman Laser which provides "eyesafe" radiation.
Many applications of lasers, such as rangefinding surveying, communication, terrain following, wire-avoidance etc. require eyesafe laser sources before they can be freely employed. One of the most successful class of solid state lasers utilises the neodymium ion as the lasing species. Although efficient, this laser normally has a wavelength around 1 .mu.m, an eye hazard at useful operating powers and repetition rates. However this can be frequency shifted to an eyesafe region of the spectrum around 1.5 .mu.m by the Raman Effect, first observed by Sir Chandrasekhara Vankata Raman in 1928.
The Raman Effect occurs when energy in the form of photons incident on a molecular structure raises the energy state of a molecule to an intermediate, or virtual state, from which it makes a Stokes transition emitting a photon of energy, termed a scattered photon. The scattered photon may have the same energy as the incident photon or alternatively a higher or lower energy, (frequency), having been "Raman shifted". For example, Raman shifting the Nd:YAG laser in methane gas generates a laser beam at 1.54 .mu.m. Likewise shifting in deuterium gas generates a laser beam at 1.56 .mu.m.
A Raman laser, employing the Raman effect, can be created by passing a laser beam, known as the pump beam, through a cell containing a Raman active medium. At lower powers the pump beam is normally focused to increase the power density within the Raman active medium, thereby enhancing the interaction, which is nonlinear, and increasing the conversion efficiency. Many other geometries are also possible, including collimated and waveguide configurations.
The Raman conversion process is typically around 40% efficient. Raman scattering is an inelastic process, i.e. energy is deposited in the Raman medium at the end of the interaction. Some 30% of the pump energy is deposited in the gas during vibrational Raman scattering in methane. The residual pump light which is unconverted exits the cell and presents a remaining eye hazard.
Guaranteeing that a Raman shifted laser is eyesafe is difficult. The design must be such that any hazardous light is eliminated or is kept below an acceptable power level at the exit of the system. This must be the case even if the Raman laser should fail for any reason, allowing the pump beam to pass through unmodified, or if the optics employed are imperfectly manufactured or get damaged.
In order to better understand the problems associated with present Raman lasers, these shall be discussed with reference to FIG. 1, which depicts a common Raman laser arrangement using longitudinal pumping. In this arrangement the laser source 1, which may be a Nd:YAG laser, produces a pump beam 2. This passes through mirror 3, (which is transmissive to the pump beam wavelength but reflective to the Raman shifted wavelength), to cell 4, containing a Raman medium 5, where part of the pump beam is frequency shifted. The Raman and residual pump beams 6 then exit the cell 4 and pass through a Raman output coupler 7, (which is a partial reflector at the wavelength of the Raman beam, and totally transmissive to the residual pump beam). This coupler is spaced relative to mirror 3 such that it forms a resonator and causes stimulated emission within the cell 4 of Raman photons. Lens element 8 is provided in order to focus the radiation within the cell to increase the conversion efficiency as mentioned above. The Raman and residual pump beams exit at 10 have been refocused by lens 9 and are then separated and filtered by dielectric coated beam splitters (not shown), and/or bulk absorbers (e.g. silicon). Such methods are less than perfect because dielectric mirrors/splitters can be imperfectly made, for example they may contain pinhole defects, and both splitters and bulk absorbers suffer stress at high incident beam intensities and may fail. Also a method known as Four Wave Mixing can generate other hazardous wavelengths in the forward direction when, as in this simple geometry, the pump and Raman beams are co-propogating. The splitters and bulk absorbers must also cope with these.
It is important to minimise intensities wherever possible in the simple longitudinally pumped Raman laser design as described above, because if the Raman laser fails for any reason (e.g. loss of gas, misalignment of Raman resonator, damage to Raman resonator optics etc.) then the pump beam is unconverted. It will be appreciated from FIG. 1, that if this happens the hazardous radiation increases some threefold at the outlet of the system, increasing stress on the output protective optics and/or bulk absorber, and increasing the risk of unwanted radiation escaping.
The object of the present invention is to provide a Raman laser which overcomes the above mentioned problems.